The disclosure relates generally to the field of wellbore drilling using a pump to lift drilling fluid out of the wellbore so as to maintain a selected wellbore pressure. More specifically, the disclosure relates to mud return pumps and methods for connecting such pumps to a drilling riser.
FIG. 1 shows an example “mud lift” drilling system using a drilling fluid (“mud”) return pump when drilling from a platform (drilling unit) at the water surface. Typically, a conductor is first driven into the water bottom in marine drilling operations. When drilling a borehole 15 from the drilling device, drilling fluid is pumped through a drill string 16 down to a drilling tool, usually including a drill bit (not shown). The drilling fluid serves several purposes, one of which is to transport drill cuttings out of the borehole. Efficient transport of drill cuttings is conditioned on the drilling fluid being relatively viscous. The drilling fluid flows back through an annulus 30 between the borehole wall, a liner 14, which is typically coupled to a riser 12 at a wellhead (not shown) proximate the water bottom and the drill string 16, and up to the drilling unit, where the drilling fluid is treated and conditioned before being pumped back down to the borehole. In many cases, this will result in a head of pressure that is undesirable.
By coupling a pump 20 to the liner 14 near the water bottom or to a drilling riser 12 at a selected level above the water bottom the returning drilling fluid can be pumped out of the annulus 30 and up to the drilling rig. The annular volume in the riser 12 above the drilling fluid may be filled with a riser fluid. Preferably, the density of the riser fluid is less than that of the drilling fluid.
The drilling fluid pressure at the water bottom may be controlled from the drilling unit by selecting the inlet pressure to the pump 20. The height H1 of the column of drilling fluid above the water bottom depends on the selected inlet pressure of the pump, the density of the drilling fluid and the density of the riser fluid, as the inlet pressure of the pump is equal to: P=H1γb+H2γs, wherein γb=the density of the drilling fluid, H2=the height of the column of riser fluid, and γs=the density of the riser fluid.
In order to prevent the drilling fluid pressure from exceeding an acceptable level (e.g. in the case of a pipe trip), the riser may be provided with a dump valve. A dump valve of this type can be set to open at a particular pressure for outflow of drilling fluid to the sea.
The following describes a non-limiting example of a method and device illustrated in the accompanying drawings, in which, as noted above, FIG. 1 is a schematic view of a fixed drilling rig provided with a pump for the returning drilling fluid, the pump being coupled to the riser section near the seabed and the riser section being filled with a fluid of a different density than that of the drilling fluid.
Reference number 1 denotes a drilling unit comprising a support structure 2, a deck 4 and a derrick 6. The support structure 2 is placed on the water bottom 8 (or the support structure may be affixed to flotation devices as is well known in the art) and projects above the surface 10 of the water. The riser section 12 of the liner 14 extends from the water bottom 8 up to the deck 4, while the liner 14 runs further down into a borehole 15. The riser section 12 is provided with required well head valves (not shown).
The drill string 16 projects from the deck 4 and down through the liner 14. A first pump pipe 17 is coupled to the riser section 12 near the water bottom 8 via a valve 18 and the opposite end portion of the pump pipe 17 is coupled to a pump 20 placed near the seabed 8. A second pump pipe 22 runs from the pump 20 up to a collection tank 24 for drilling fluid on the deck 4.
A tank 26 for a riser fluid communicates with the riser section 12 via a connecting pipe 28 at the deck 4. The connecting pipe 28 has a volume meter (not shown). Preferably, the density of the riser fluid is less than that of the drilling fluid.
The power supply to the pump 20 may be via an electrical or hydraulic cable (not shown) from the drilling unit 1. The pressure at the inlet to the pump 20 is selected from the drilling unit 1. The pump 20 may be electrically driven, or may be driven hydraulically by means of oil that is circulated back to the drilling unit or by means of water that is dumped in the sea from the pump power outlet.
The drilling fluid is pumped down through the drill string 16 in a manner that is known in the art, returning to the deck 4 via an annulus 30 between the liner 14 and the drill string 16. When the pump 20 is started, the drilling fluid is returned from the annulus 30 via the pump 20 to the collection tank 24 on the deck 4. Using such a system it is possible to achieve, for example a significant reduction in the pressure of the drilling fluid in the borehole 15.
A particular issue with such systems is possibility of collapse of the first pump pipe 17 as a result of differential pressure between the hydrostatic pressure of the water at the depth of the pump pipe 17 and the internal pressure of the first pump pipe, depending on the pressure desired to be maintained in the wellbore. This is particularly an issue when the first pump pipe is made of flexible material, such as rubber hose. Such flexible materials are used so that the location of the pump 20 may be moved to suit the particular conditions in the water or proximate the water bottom 8.
What is needed is a pump system that excludes the use of a lengthy first pump pipe between the riser outlet and the pump inlet.